feat: add GitHub Action to build benchmark APK on claude branches

Adds a lightweight workflow that triggers on pushes to claude/* branches,
building only the Play Benchmark APK for quick testing after Claude
completes a task.

https://claude.ai/code/session_013hcKzBgJgfsrArSgTTGNEk

.github/workflows/build-benchmark-apk.yml

@@ -0,0 +1,43 @@
+name: Build Benchmark APK
+
+on:
+  push:
+    branches:
+      - 'claude/**'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-benchmark:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v5
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Cache gradle
+        uses: actions/cache@v5
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
+      - name: Build Benchmark APK
+        run: ./gradlew assemblePlayBenchmark
+
+      - name: Upload Play Benchmark APK
+        uses: actions/upload-artifact@v6
+        with:
+          name: Play Benchmark APK
+          path: amethyst/build/outputs/apk/play/benchmark/amethyst-play-universal-benchmark.apk